Finned tube heat exchangers are commonly used in refrigeration and air-conditioning systems. A typical heat exchanger, for purposes of discussion, comprises a bank of spaced, parallel thin metal plates having holes formed in them to accept parallel runs of hollow metal tubes, typically aluminum or copper. When joined with the tubes, the plates become fins which greatly increase the surface area available for thermal transfer between the fluid in the tube and the air contacting the fins. The tubes are joined with one another to provide a continuous fluid passage for a fluid or refrigerant between an inlet and an outlet. The heat exchanger may be an evaporator or a condenser.
One method of constructing such a heat exchanger involves passing elongated U-shaped sections of tubing, known as hairpins, through the holes formed in the fins. The hairpins are then interconnected at their open ends with short U-shaped tubes called return bends, so as to form a zigzag flow path through the tubing. The return bends are usually connected to the hairpins by brazing, an operation that must be closely controlled to assure a high quality product.
Another method of constructing such a heat exchanger involves bending a single, continuous length of tube into a zigzag pattern, or serpentine, with parallel tube runs connected to one another by constant radius 180 degree bends at either end. The fin plates are formed with a regular pattern of elongated slots and arranged in a fin bank with the slots in alignment. The serpentine is then inserted, or "telescoped," into the fin pack, with the bends at one end of the serpentine passing completely through the slots of the fin pack, leaving the straight runs of tubing disposed in the ends of the slots. This method has the advantage of requiring fewer brazed joints and is compatible with automated production equipment. The method and the resulting product are described in U.S. Pat. No. 3,345,726 to Charles Hickman.
Another method of constructing a finned tube heat exchanger with a continuous serpentine is shown in U.S. Pat. No. 4,625,378 to Tanno et al. In this construction, small fin plates having only two through-punched holes are arranged in a fin bank, and the straight portions of a single long hairpin are passed through the aligned holes. The resulting structure is then bent into a zigzag configuration to give the desired number of tube passes. A heat exchanger made by this method is limited to having only two rows of tube passes.
Both the Hickman and Tanno methods are limited in that, at least as applied in an automated high volume production process, they produce a heat exchanger having a uniform, evenly spaced pattern of tube runs. This is a consequence of the fact that the machinery which bends the tubing into a serpentine can only easily produce bends of a single, predetermined radius. Thus, each run of tubing is separated from its neighbors by a distance equal to twice the bend radius.
In many heat exchanger applications it is desirable to have a fluid flow path other than the regular pattern described above. By varying the distance between the tube passes it is possible to generate different temperature and pressure gradients within the fin bank, alter the air flow, and change the air pressure drop, frost load, and heat absorption. All of these factors relate directly to the performance and efficiency of a refrigeration system. In the past, the only way to construct a heat exchanger with such a customized, non-regular layout has been with the brazed hairpin construction described above.